(Zetterstedt, 1838)
Subgenus Euorthocladius Thienemann, 1935
Body
Medium sized larvae up to 8 mm long. Larvae are pale brownish. The L4 setae are simpel, appr. 200 µm long and about 1/3 of the segment length. The anterior and posterior parapods are separate and bear rows of claws. Procercus and two pairs of anal tubules present (Wiederholm, 1983; Moller Pillot, 1984b).
Head
Headcapsule pale to brown-yellow with a darker hind margin. Head length 0.43 - 0.66 mm (Moller Pillot, 1984b; Schmid, 1993).
Antenna
The antenna consists of five segments. The blade is shorter than the flagellum. Lauterborn organs are small.
Mentum
The mentum has one median tooth and 6 pairs of lateral teeth. The mentum ratio is 1.3 - 2.0 (Schmid, 1993) Euorth rivi mentum. The ventromental plates are weak. The beard is absent (Wiederholm, 1983).
Labrum
The pecten epipharyngis consists of three scales, SI is bifid and premandible with one apical tooth Orth frigidus labrum9 (Wiederholm, 1983; Moller Pillot, 1984b).
Mandible
The mandible has an apical tooth of normal length and 3 inner teeth Orth frigidus mandible. The seta interna is absent (Moller Pillot, 1984b).
Differential characteristics
Orthocladius can be separated from all other genera, except Paratrichocladius, by the bifid SI, mentum with 1 median tooth and 6-9 pairs of lateral teeth, 5 segmented antenna (4 in O. consobrinus) and simpel body seta. Paratrichocladius however has a blueish thorax in freshly preserved larvae and a lobed ring beneath the ssd of the mandible. Both characters are absent in Orthocladius.
Orthocladius frigidus can be separated from the other Orthocladius with a pale to light brown head by:
L4 appr. 200 µm, mentum with 6 pairs of lateral teeth and head length > 0.42 mm.